Such a method is of particular importance for measuring radioactive radiation with a scintillator. Therefore, in what follows, scintillation measurements will be the example which serves as a basis for explanations and discussions of the problem setting.
Scintillation measurements are one of the best known and oldest methods for detecting radioactive radiation of the different types and from different sources, for example, alpha, beta or gamma radiation, and also neutrons. In the reference by Glenn F. Knoll "Radiation Detection and Measurement", 2nd. Ed., John Wiley & Sons, 1989, page 215 et seq., the bases and measuring processes are extensively described. Reference will be made in what follows to individual sections of this reference which are of special interest here.
For measuring radioactive radiation, radiation arriving in the scintillator is used to raise electrons to a higher energy level, from where they subsequently revert (in accordance with a decay time) back to the original, non-exited state (basic state) while emitting photons. The photons generated in this manner then trigger electrons in an opto-electronic detector associated with the scintillator, the electrons are multiplied in the detector so that discrete output pulses, which are superimposed to a greater or lesser degree, are generated for each photoelectron, and these pulses can be registered following further external amplification.
Depending on the incidence energy of the radioactive radiation to be detected, the latter is able to excite a certain number of molecules in the scintillator in this way, and as a consequence a number of photons will act in a statistical sequence on the detection plane of the opto-electronic detector (for example the photo-electric layer of a photo-multiplier, an avalanche photo-diode or a microchannel plate amplifier) in order to trigger electrons, namely the photoelectrons, there.
When using a photo-multiplier as the opto-electronic detector, it is basically true that after passage through the photo-multiplier dynodes (amplifier stages), individual photoelectrons generate output signals in an approximately symmetrical temporal progression. Although their amplitude depends on the amplification process of the dynodes, their pulse shape with a defined half-width value is essentially constant.
If therefore a number N of molecules is excited in the scintillator by the incident radiation to be detected, the pulses resulting from the generated photoelectrons are superimposed to form a signal the integral value of which is a measurement of the energy of the incident radiation to be measured.
If this is displayed in a pulse height spectrum, with high energy-reach radiation a peak at relatively large pulse heights is obtained.
However, a signal progression at the detector output may also occur during a scintillation measurement which is caused by only a few photoelectrons, namely when the energy of the radiation to be detected is very low or the yield of the scintillator is very low.
Such events therefore cause a signal progression with considerably lower pulse height, which in the pulse height spectrum is distributed among small amplitude values.
In the pulse height spectrum such pulses then overlap those generated by radiation which is not to be detected and is generated by individual photoelectron events. Such single photoelectron events are caused by the thermal emission of electrons from the photo-cathode, for example, or by the emission of individual photons from excited states with long decay times of the scintillator (so-called afterglow).
A highly sensitive measurement of weakly energetic radiation can be considerably impaired by this (Knoll, page 259/260). It is decisive that, depending on the electron amplification, in particular of the first dynode of the photo-multiplier, relatively large half-width values appear in the pulse height spectrum. The direct consequence of these conditions is:
a. When the radiation to be detected is of sufficiently high energy to trigger at least approximately four electrons in the photo-sensitive layer of the photo-multiplier, a sufficient distance between the above mentioned single photoelectron peaks (caused by the so-called interference pulses) on the one hand, and the multiple photoelectron peak (as usable signal) on the other hand will be present in the pulse height spectrum in spite of the system-specific half-width values, and the unwanted single photoelectron events can be easily eliminated by pulse height discrimination. PA1 b. However, if the radiation to be detected is low in energy (such as with tritium as a beta radiation source or with 55 Fe as a gamma radiation source), so that the pulse amplitude reaches the order of the pulse amplitudes of single photoelectrons, this can result in that the two peaks in the pulse height spectrum intermingle to such an extent that pulse height discrimination is unsuitable for detecting a difference between the ionizing radiation signal and the "single photoelectron noise". PA1 creating a time window having a selected duration commencing with the beginning of each signal at the output; PA1 determining a parameter value of each signal dependent on the time coordinate of the center of gravity of the signal within the time window; and PA1 comparing the parameter value with a reference value corresponding to the parameter value for a single electron event.
As known from the cited reference, for photoelectron events the width of the peak essentially depends on the secondary electron amplification .delta. of the first dynode of the photo-multiplier. The overlap of the peaks mentioned, which prevents a clear discrimination, can therefore be reduced if, when a photo-multiplier is employed, the first dynode has a particularly high secondary electron amplification .delta.. This can be achieved, for example, by the use of so-called NEA material for the first dynode. With this it is possible to achieve .delta. amplification factors of 15 to 20, while standard dynodes only have a .delta. amplification factor of approximately 4 to 6.
An opportunity is provided by this to reduce the half-width values of a five-photoelectron event and a single photoelectron event, for example, at a correspondingly greater constructive expense, sufficiently so that it becomes possible to set a reasonable threshold for pulse height discrimination. However, the improvement achieved by means of NEA dynodes is not sufficient to separate an event resulting from two electrons from a single photoelectron event. As a result of the noise of the photo-cathode and a large number of single photons from the scintillator, the number of single electron events is so high that the resulting peaks of the single photoelectron events and the multiple photoelectron events overlap to such an extent that it is no longer possible to set a meaningful discrimination threshold.
A further improvement in the sense of a separation of single photoelectron events from multiple photoelectron events can be achieved through a further increase, namely by means of a coincidence measurement, for which two photo-multipliers are spatially associated with the scintillator. In the course of this only those pulses are evaluated which, within a preset time window, are registered by the first as well as the second photo-multiplier. In this way it is assuredly possible to exclude single photoelectron events. However, for statistical reasons the detection sensitivity of such an arrangement also decreases with the decrease in energy of the radiation to be detected. With radiation to be detected which generates two photoelectrons, there is a 50% probability that the second photoelectron is generated in the same photo-multiplier as the first, so that this event is not registered although it stems from the radiation to be detected.
Furthermore, the use of two photo-multipliers, for example in a portable detector for measuring dose rate, is very awkward. In addition, the two photo-multipliers screen each other in respect to external radiation so that such an arrangement is only usable in a limited manner in case of measuring external X-ray radiation, for example.
Several solutions have been proposed in various issues of the periodical "Nuclear Instruments and Methods" (NIM) for solving these problems, all of which utilize preset discrimination criteria between single electron events and multiple electron events, after which they provide a more or less dependable discrimination with the goal of eliminating the "single electron noise" by suitable discrimination methods and corresponding circuitry:
NIM 33 (1965), pp. 303 to 305, describes the use of rise time of pulses of the photo-multiplier noise as the discrimination criterion, which is said to be considerably less than with useful signals of the scintillator.
NIM 120 (1974), pp. 61 to 68 describes use of pulse length as the discrimination criterion, and NIM 71 (1969) pp. 173 to 186, suggests to compare the amplitudes of an integrated dynode output signal with the amplitude of a discriminated anode output signal in order to detect electron events and to eliminate them.
All these methods may operate dependably when the pulses to be discriminated have sufficient time spacing from each other, however with overlaps there no longer is a dependable discrimination.
A similar situation arises with the method in accordance with European Patent Disclosure EP 0 425 767 A1 wherein, for discriminating between single electron event and a multiple electron event, a check is made by means of a digital circuit whether within a preset time window one or several discrete single pulses occur. If only a single pulse occurs, the result is excluded from the subsequent evaluation as being a noise pulse, while a useful signal is identified by the occurrence of several single pulses.
This already causes the difficulty, as described at col. 5, lines 7, et. seq. of the above-cited European Patent, that it is necessary to take additional technical circuit measures to detect a signal progression which appears to be a single pulse, but is the result of a multiple electron event (and thus a true scintillation event), and to admit it for evaluation. For this purpose a sequence of "replacement pulses" is generated during the length of this pulse progression above a certain threshold, which so to speak simulate the multiple electron event "hidden" in the signal progression.
Aside from additional cost, the proposed solution with its digital circuit concept can only be used in a limited way, because a signal progression only allows the conclusion that there had been two pulses (and thus a "two-photoelectron event") if the distance between these pulses is at least 10 ns (col. 6, lines 40 et seq of the European Patent.). This therefore limits the range of utilization of the known method to scintillators with decay times of at least 15 ns. Thus this method is unsuited for the most frequently used plastic and fluid scintillators, the decay times of which mostly are below 5 ns.